The invention relates to paper machine clothing useful for fabrication of dryer felts and the like which are employed in the dryer section of a papermaking machine to dry paper in web or sheet form.
In the papermaking machine, an aqueous suspension of fibers is transformed in stages into a paper web which is passed about an array of heated cylinders in order to dry the paper web. As the paper web passes over the dryer cylinders, it is held in contact with the cylinders for drying by a belt, commonly referred to as a dryer felt or fabric which has been made endless by techniques well known in the art.
The dryer fabrics are typically woven, nonwoven, or of a needle construction. The paper web dries from the heated cylinder outwardly through the dryer fabric which presses the paper web in intimate contact with the heated cylinder. Thus, the dryer fabric must possess a sufficient amount of permeability in order that the fabric breathe and allow the paper web to dry. The permeability characteristic of the dryer fabric is also important in reducing paper flutter. This is often caused through a pumping action of the air surrounding the moving fabric which causes the paper to leave the fabric momentarily. If the fluttering becomes excessive, the paper will be marked when it strikes the fabric upon returning. The permeability characteristic of the fabric is also important in regards to the type of paper which is being dried on the fabric.
Because of the conditions under which the dryer fabric is used, the dryer fabric accumulates various particulate matter from the papermaking process which, if left to accumulate in the open mesh of the fabric, can affect the permeability of the fabric. The fabric must then be cleaned by various processes well known in the papermaking art.
Fabrics constructed from monofilament material have been recognized for their cleanability and are highly desirable from this standpoint. However, the provision of a monofilament fabric with a low permeability characteristic is a problem due to the stiffness of the monofilament strands compared to spun and multi-filament fibers. In the weaving of monofilament, void spaces in the fabric are created by the weaving of the relatively stiff monofilament strands and inability of the monofilament warp strands to conform to the shape of the weft strand which leaves the fabric open and limits the low permeability range of the fabric.
Conventional fabric woven from multi-filament strands suffer from lack of cleanability due to the fibrous affect of the fibers and moisture absorptivity thereof. Moreover, it has always been thought that multi-filament wire is less desirable in papermaking fabrics since it was believed that such fabrics had less dimensional stability in the cross-machine direction due to the tendency of the elements of the fibrous material in a multi-filament yarn to slide over one another.
Attempts to close up monofilament fabric and provide a lower permeability characteristic in the fabric has been made by weaving a multi-layer fabric in which an intermediate layer includes stuffer filling yarns such as in U.S. Pat. No. 4,274,448. The stuffer yarn is a monofilament or multi-filament core with a sheath made from asbestos or other soft material. The desirable cleaning characteristics of monofilament fabric are thus not entirely provided. While the stuffer filling yarn provides a closer fit in the intermediate layer, the yarns are also more difficult to weave owing to their size and nature. When placing the stuffer yarns under tension during weaving, care must be taken that the sheath does not slide over the core material.
U.S. Pat. No. 4,224,372 discloses a multi-layered dryer fabric which includes an intermediate layer in which a core yarn is woven in the filling direction which has a coating of a thermoplastic resin foam forming composition. After weaving, the fabric is heated which causes the coating composition to foam whereby empty spaces in the intermediate layer are filled up by the closed cell foam material formed in situ which binds with the remaining fabric structure. After repeated use under high temperatures in the high temperature environment of a dryer section of a papermaking machine, the probability exists of the resin binder breaking down and disassociating with the fabric structure causing changes in the fabric permeability and fabric stability.
U.S. Pat. No. 4,351,874 discloses a low permeability dryer fabric woven from monofilament plastic warp and weft strands wherein a weft stuffer strand is utilized which exhibits preferential softening under the influence of heat. The strands soften to adapt and conform to mesh interstices and thereby restrict the passages of air through the fabric. However, in the case of most monofilament, the monofilament stuffer is relatively stiff and hard to weave leaving significant pre-heating void areas in the weave and a fairly open mesh fabric after weaving. Therefore, even with heat softening of the filler after weaving, the closing of the fabric and permeability in the low range is limited.
Accordingly, an important object of the present invention is to provide a dryer fabric for a dryer section of a papermaking machine which is constructed from a monofilament material affording ease of cleanability yet has a low permeability characteristic.
Another important object of the present invention is the provision of a woven low permeability dryer fabric having a filling element woven in the fabric subjected to melt during heat treatment to close the mesh openings in the fabric yet which is textured soft for weaving to provide a more closed pre-melt mesh whereby a very low permeability characteristic is afforded.
Still another important object of the present invention is to provide a dryer fabric for a papermaking machine constructed from polymeric monofilament material in which open spaces in the sheds are closed by multi-filament weft yarns consisting of twisted low-melt monofilament strands interlocked in the fabric by heat expansion.
Still another important object of the present invention is to provide a woven monofilament multi-layered dryer fabric in which a low-melt polymeric multi-filament weft element is woven in intermediate layers to afford more pre-melt closure of void areas in the sheds whereafter the multi-filament weft element is heated and expands further filling the void areas in the intermediate fabric structure.